An acousto-optical filter is one example of an acousto-optical polarization converter. In such a converter, a transducer of interdigitated electrodes is formed at the surface of piezo-electrical material and is electrically driven by an RF-frequency signal to launch an acoustic wave at the surface of the material. The surface acoustic wave acts as a periodic index grating for input optical radiation, and it provides for quasi-phase-matched conversion between orthogonally polarized eigenstates having substantially different refractive index. The interaction rotates the polarization of that wavelength of light for which the momentum mismatch between polarization states nearly exactly matches the acoustic wave momentum. Placing the converter between crossed broad-band polarizers allows it to operate as a narrow-band acousto-optical tunable filter (AOTF).
Many of the early AOTFs relied on bulk acoustic waves and dissipated large amounts of RF power. More recent devices have reduced the power levels by launching only a surface acoustic wave, by using an acoustic waveguide to guide the surface acoustic wave along only a limited cross-section of the surface, and by using optical waveguide so that the surface acoustic wave need only interact with the light over a limited cross-section. By combining these refinements, the RF power consumption of AOTFs has been reduced to less than 10 mW/channel, allowing for their practical use in many applications. For example, an AOTF is envisioned for filtering one or more channels of a wavelength-division multiplexing (WDM) optical communication system. Filtering of multiple channels multiplies the RF power applied to the AOFT, thus increasing the thermal problems as well as severely limiting the channel capacity. Finally, for commercial telephone usage, the RF power must be minimized to reduce cost.
Acousto-optical converters further suffer from high-intensity frequency side lobes. Although the central lobe can be made exceedingly narrow, the intensity of the side lobes remains proportionally constant in most designs. For a single-stage abrupt turn-on AOTF, the first side lobe is typically reduced by only 10 dB from the resonance. High side lobes reduce the filtering effectiveness by causing optical leakage between neighboring wavelength channels, and they impose design constraints on the use of an AOTF in a WDM system. Cheung et al. disclose in U.S. Pat. No. 5,002,349, incorporated herein by reference, a multi-stage AOTF having reduced side lobes. However, the serially connected multiple stages need to be acoustically isolated from each other. Therefore, their AOTF needs multiple transducers and extends over a substantial length. This design increases cost and introduces processing variations between different portions of the AOTF. Furthermore, a severe type of crosstalk, called coherent crosstalk, is not significantly reduced by serially connecting two filters with high side lobes.
Fowles defines apodization in the text Introduction to Modern Optics, 2nd ed. (Holt, Rinehart and Winston, Inc., 1975), pp. 138-139 as "any process by which the aperture function is altered in such a way as to produce a redistribution of energy in the diffraction pattern." He shows that the diffraction pattern through an apodized slit reduces spatial side lobes. Morgan discusses apodized transducers in surface-wave devices in his treatise Surface-Wave Devices for Signal Processing (Elsevier, 1985), pp. 61-64. His apodized launching transducer has interdigitated electrodes having an overlap between neighboring electrodes that varies along the direction in which they launch the surface acoustic wave. The frequency response of the device depends on the details of the apodization. Alferness discloses an optical directional coupler in "Optical directional couplers with weighted coupling," Applied Physics Letters, volume 35, 1979, pp. 260-262 in which two optical waveguides forming an optical directional coupler approach each other across a precisely chosen gap or interaction region that varies in a carefully chosen manner. He is thereby able to reduce the size of the frequency side lobes. He obtains his best results with a Hamming function taper of the gap although raised cosine tapering is also effective.
Yamamoto et al. propose an apodized acousto-optic converter in "Guided-Wave Acoustooptic Tunable Filters Using Simple Coupling Weighting Technique," Proceedings of 1990 IEEE Ultrasonics Symposium, 1990, pp. 605-608. The apodization is achieved by tapering the acoustic waveguide, in the middle of which runs an optical waveguide. The varying cross-section causes the acoustic energy density in the acousto-optic interaction region to begin at a small value, increase slowly to a maximum value, and thereafter decrease. However, we have determined that this technique is difficult. If the acoustic power is to be gradually concentrated in the narrowing acoustic waveguide, the acoustic wave must be adiabatically compressed or else a great deal of energy is lost as higher-order modes exceed the waveguide cutoff frequency and leak into the substrate. Adiabatically tapered waveguides have proven difficult to fabricate.